As methods of detecting wire breaking, variety methods thereof have been proposed hitherto, for example, such as a method of performing a wire breaking detection by detecting a phase current in a load (motor) and a method of performing a wire breaking detection while providing an operating state used for the wire breaking detection prior to starting a motor.
As the above method of detecting the motor phase current, patent JP-A-2001-305175 discloses a method of performing a wire breaking detection based on a ripple variation amount in a phase current. Further, as the method of providing the operating state used for the wire breaking detection prior to starting the motor, JP-A-2007-143244 discloses a method of performing a wire breaking detection by detecting a current flowing through a DC side in an electric power conversion circuit while applying a DC voltage thereto prior to starting the same. Still further, JP-B-3801471 discloses a method of performing a wire breaking detection only under a certain condition (when an angular velocity of a motor is above a predetermined value and a current command value of the motor is above a predetermined value).
An example of the above method of detecting the motor phase current will be explained with reference to FIGS. 16 and 17. FIG. 16 shows a schematic constitutional diagram of a motor drive device and FIG. 17 shows current variations (ripples) at a normal condition and at a wire breaking condition. In the present example, as the motor a DC motor 90 with brushes (a positive pole brush 95a and a negative pole brush 95b) and a commutator is exemplified. FIG. 17(a) shows a current variation (ripple) in a normal instance. In the drawing, an amount of current variation (difference between the maximum value and the minimum value) gives I−1. On the other hand, FIG. 17(b) shows a current variation at an instance when a motor winding in a certain phase is wire broken. In the drawing, the amount of current variation gives I-2, which is larger in comparison with that at the normal condition (FIG. 17(a)). A control circuit 94 can detect a wire breaking in the motor windings by comparing an amount of current variation detected by a current sensor 93 with a judgment reference value stored in advance.
However, with this method it is indispensable to set the judgment reference value at a proper value. For this reason it is necessary to alter the judgment reference value depending on such as size of the load and ambient temperature. Namely, it is difficult to apply the method to a usage where the load condition greatly changes. Further, since the judgment is performed based on the amount of current variation, the method cannot be applied in association with current detection accuracy under a condition when a certain amount of current is flowing. Still further, the method cannot judge which phase in the motor windings is wire broken.
Now, an example of the method of providing the operating condition used for the wire breaking detection prior to starting the motor will be explained with reference to FIG. 18. In the present example, the wire breaking detection is performed by detecting a current flowing through a shunt resistor 97 disposed at a DC side of an electric power converter circuit 98 at a moment when a DC voltage 99 is applied to a motor 96 prior to starting the motor 96. In order to apply the DC voltage 99 to the motor 96, for example, as shown in FIG. 18(a), an upper switching element (U+) in U phase and lower switching elements (V− and W−) in V phase and W phase are turned on and the other switching elements are turned off. At this moment, a current flows in the electric power conversion circuit 98 and the motor 96 through a route indicated by arrows in the drawing. Herein, when a motor winding in U phase is wire broken, no current flows through the shunt resistor 97. Accordingly, the wire breaking detection is performed by detecting a current flowing through the shunt resistor 97. However, if a wire either in V phase or in W phase is broken, since a current flows through a route in the other phase, in this instance the wire breaking detection with this current route fails. Therefore, it is necessary to detect successively the current flowing through the shunt resistor 97 while changing the switching state of the respective phases (for example, as such as in FIG. 18(b) and FIG. 18(c)).
In the present example, since the current detection during the time when the DC voltage is applied is preconditioned, the wire breaking detection can only be performed prior to starting the motor. In other words, the wire breaking detection cannot be performed during the operation of the motor. For this reason, in an application where a motor is once started the motor is continuously driven long time, it takes time until a wire breaking detection is performed. Further, since the wire breaking detection has to be performed by successively changing the switching state, it is difficult to apply the method to an application that requires, for example, to start a motor in a short time.
As has been explained above, problems of the conventional methods of detecting the motor phase current and of providing the operating state used for the wire breaking detection prior to starting are such as necessity of the current flowing of a certain amount, the limited application conditions and the incapability of wire breaking detection during operation. Further, the technology disclosed in patent document 3, is for a method of detecting wire breaking only under a certain condition, therefore, the wire breaking detection therewith cannot be performed without regarding operating conditions.